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Computer
systems have a finite amount of physical memory that is made available to the
operating system. When the operating system begins to approach the limit of the
available memory it frees up space by writing memory pages to disk. When any of
those pages are required by the operating system they are subsequently read
back into memory. The area of the disk allocated for this task is referred to
as swap space.

Recommended Swap Space for CentOS 6

The
amount of swap recommended for CentOS 6 depends on a number of factors
including the amount of memory in the system and the workload imposed on that
memory. The current guidelines for CentOS 6 swap space are as follows:

§4GB of RAM requires a minimum of 2GB of swap space

§4GB to 16GB RAM requires a minimum of 4GB of swap space

§16GB to 64GB of RAM requires a minimum of 8GB of swap space

§64GB to 256GB of RAM requires a minimum of 16GB of swap space

Identifying Current Swap Space Usage

The
current amount of swap used by a CentOS 6 system may be identified in a number
of ways. One option is to cat the /proc/swaps file:

# cat /proc/swaps

Filename Type Size Used
Priority

/dev/dm-1 partition 2064376 0 -1

Alternatively,
use the swapon command:

Filename Type Size Used
Priority

/dev/dm-1 partition 2064376 0 -1

Finally,
the free command may also be used:

# free

total used free
shared buffers cached

Mem: 1021488 615348
406140 0 199596
160312

-/+ buffers/cache:
255440 766048

Swap: 2064376 0
2064376

Adding a Swap File to a CentOS 6 System

Additional
swap may be added to the system by creating a file and assigning it as swap.
Begin by creating the swap file using the dd command (the size can be changed
by adjusting the count= variable; the following creates a 131MB file):

# dd if=/dev/zero of=/newswap bs=1024 count=128000

128000+0 records in

128000+0 records out

131072000 bytes (131 MB) copied, 1.7639 seconds, 74.3 MB/s

Configure
the file as swap:

# mkswap /newswap

mkswap: /newswap: warning: don't erase bootbits sectors

on whole disk. Use
-f to force.

Setting up swapspace version 1, size = 127996 KiB

no label, UUID=91588fb6-ac01-47f6-bb42-064c1c7c4599

Add
the swap file to the system in real-time:

# swapon /newswap

Finally,
modify the /etc/fstab file to automatically add the new swap at system boot
time by adding the following line:

/newswap swap swap
defaults 0 0

Adding Swap to a CentOS 6 LVM Swap Volume

By
default, CentOS 6 configures swap space using Logical Volume Management (LVM).
An alternative to adding swap via file, therefore, is to extend the logical
volume used for the swap.

The
first step is to identify the current amount of swap available and the volume
group and logical volume used for the swap space (for more information on LVM,
refer to the chapter entitled Adding a New Disk to a CentOS 6 Volume Group and
Logical Volume):

# lvdisplay

--- Logical volume ---

LV Name /dev/vg_centos6/lv_root

VG Name vg_centos6

LV UUID
nU70n0-scyi-xp2S-1CNq-GUa1-4Vad-gQKIOO

LV Write Access read/write

LV Status available

# open 1

LV Size 23.54 GiB

Current LE 6026

Segments 2

Allocation inherit

Read ahead sectors auto

- currently set to 256

Block device 253:0

--- Logical volume ---

LV Name /dev/vg_centos6/lv_swap

VG Name vg_centos6

LV UUID
5uVonb-SCk2-bedt-4Oj3-LGpq-TpsA-qyCGEl

LV Write Access read/write

LV Status available

# open 1

LV Size 1.97 GiB

Current LE 504

Segments 1

Allocation inherit

Read ahead sectors auto

- currently set to 256

Block device 253:1

Clearly
the swap resides on logical volume lv_swap which in part of volume group
vg_CentOS6. The next step is to verify if there is any space available on the
volume group that can be allocated to swap volume:

# vgs

VG #PV #LV #SN Attr VSize
VFree

vg_centos6 2
2 0 wz--n- 47.50g 22.00g

If
the amount of space available is sufficient to meet additional swap
requirements, turn off the swap and extend the swap logical volume to use the
additional space:

In
the above section we extended the swap logical volume to use space that was
already available in the volume group. If no space is available in the volume
group then it will need to be added before the swap can be extended. Begin by
checking the status of the volume group:

# vgs

VG #PV #LV #SN Attr VSize VFree

vg_centos6 1
2 0 wz--n- 8.79g 0

The
above output indicates that no space is available within the volume group.
Suppose, however, that we have a requirement to add 2GB to the swap on the
system. Clearly, this will require the addition of more space to the volume
group. For the purposes of this example it will be assumed that a disk
partition that is 2GB is size and represented by /dev/sdb is available for
addition to the volume group. The first step is to turn this partition into a
physical volume:

# pvcreate /dev/sdb

Physical volume
"/dev/sdb" successfully created

Next,
the volume group needs to be extended to use this additional physical volume:

# vgextend vg_centos6 /dev/sdb

Volume group
"vg_CentOS6" successfully extended

At
this point the vgs command should report the addition of the 2GB of space to
the volume group:

# vgs

VG #PV #LV #SN Attr VSize
VFree

vg_centos6 2
2 0 wz--n- 10.78g 2.00g

Now
that the additional space is available in the volume group, the swap logical
volume may be extended to utilize the space. First, turn off the swap: